Test objects requiring inspection for detecting defects include, for example, porous members. Porous members are extensively used for filters or catalyst carriers, etc. Porous members are used, for instance, for the exhaust gas purifiers of heat engines, such as internal combustion engines, or combustion equipment, such as boilers, for reformers of liquid fuel or gas fuel, and for purifying facilities of water supply and sewerage. Furthermore, honeycomb-shaped porous members are used for diesel particulate filters or hot gas dust collectors to capture and remove particulate substances contained in a dust containing fluid, such as exhaust gas, emitted from a diesel engine.
A porous member used for such purposes mainly functions to capture and remove unwanted particulate substances when a fluid to be treated passes through micro-pores of the porous member, or to carry a catalyst on the surface and micro-pores of the porous member to bring the catalyst into contact with the fluid to be treated. In order to efficiently implement such functions, a porous member made of a thin film or wall is generally formed to have a shape, such as a tubular shape, a monolithic shape, or a honeycomb structural shape so as to increase the area that is brought into contact with a fluid to be treated. Therefore, the presence of a hole, i.e., a defect, that pierces the film or wall of a porous member interferes with the filtering performance of the porous member or the performance as a catalyst carrier. In the case of a nonporous member, a hole, that is, a defect, frequently prevents the performance expected of the material from being fully exhibited. The simplest method for inspecting such porous members or nonporous members for defects is visual inspection.
There are, however, some cases where the inspection is difficult, depending on the configuration, or the position or the size of a defect to be detected, of a test object, which may be a porous member or a nonporous member. For example, a honeycomb structure generally has numerous passages through the honeycomb structure in the axial direction (X direction), the passage is defined by partition walls, as shown in FIG. 8. In some cases, as shown in FIG. 2, the ends of the passages are alternately sealed, and the porous partitions capture and remove particulate substances. Hence, defects in the partitions of such a structure cannot be observed from outside.
There has been a known conventional method for inspecting a defect of such a test object, in which the upper end face of a honeycomb structure shown in, for example, FIG. 8 is covered with a white cloth, and soot-like substances generated by burning diesel fuel light oil is introduced from the lower end face of the honeycomb structure to cause the soot-like substances discharged from the upper end face to adhere to the white cloth, thereby obtaining the pattern shown in FIG. 7. This method for detecting a defect is called “the soot print method”. In this case, the presence of a defect is indicated by a dark spot where more soot adheres. This method is handy and excellent in detecting the level of a defect and identifying the location of the defect. However, since the method involves the use of a soot-like substance, the soot-like substance that adheres to a honeycomb structure must be removed by heat treatment after inspection, and a few hours are required for such post-treatment. In addition, inspecting one test object requires 5 to 6 minutes.
There is another method in which a honeycomb structure is placed in water, and an air pressure is applied from one end face of the honeycomb structure to check for the presence of a defect from foaming on the other end face. The post-treatment of this method does not take as long as that of the aforesaid soot print method; however, the honeycomb structure has to be dried after the inspection, and also has to be de-foamed in water prior to the inspection, which takes time.
In making a porous member, there are some cases where a material is formed into a compact in a predetermined shape, then fired to produce pores thereby to turn the compact into the micro-porous member. Therefore, if it is possible to detect a defect already existing in the nonporous member prior to the firing process, it is economically advantageous because a defective member can be removed before it is fired.
In view of the above circumstances, an object of the present invention is to provide an inspection method and an inspection apparatus that may detect a defect with good sensitivity even if a test object has a shape that prevents a defect from being checked from outside or whether a test object is porous or nonporous.
Another object of the present invention is to provide an inspection method and an inspection apparatus that may identify a location of a defect easily.
Still another object of the present invention is to provide an inspection method and an inspection apparatus capable of recording a location of a defect easily.
A further object of the present invention is to provide an inspection method and an inspection apparatus with short inspection time and short or no pre-treatment and/or post-treatment time.